Automatic gain control



June 24, 1941. QC. MARTINELLI 2,246,947

AUTOMATIC GAIN CONTROL Filed May 28, 1938 I 2 Sheets-Sheet 2 +100 mv. c. W

lnventor aim C.MartineI/Zi W attorney Patented June 24, 1941 AUTQMATIG GAIN CONTRQL Ciro C. Martinelii, Oaklyn, N. J., assignor to Radio Corporation of America, a corporation of Dela- Application May 28, 1938, Serial No. 210,614

(Cl. ITS-7.3)

6 Claims.

My invention relates to automatic gain control or A. V. 0. systems and particularly to A. V. C.

systems for use in television receivers or the like.

It has been found desirable to so design television transmitters that the direct current component of the picture signal is transmitted whereby the transmitter tubes are utilized to transmit the maximum power which their rating permits. When the D.-C. component is transmitted, the amplitude of the transmitted carrier varies in accordance with the D.-C. component. Thus, the carrier amplitude is not a true measure of the attenuation or fading of the transmitted signal and a conventional A. V. C. system is not satisfactory.

It is, accordingly, an object of'my invention to provide an improved automatic gain control system for television or picture receivers.

It is a further object of my invention to provide an improved system for automatically controlling the gain of a television receiver which is receiving signals from a transmitter of the type that transmits the direct current component of the picture signal.

It is a still further object of my invention to provide for use in a television system transmitting direct current an improved automatic gain control circuit which will not become ineffective and thus permit the amplifier tubes of a receiver to overload as a result of the modulation being taken off the carrier and then being put back on.

In Holmes Patent No. 2,109,618 there is described an A. V. C. system for a television receiver in which the volume or gain control voltage is derived from the synchronizing impulses rather than from the carrier wave. In accordance with my invention, I make use of the broad principle described in the Holmes patent, that is, the gain control voltage is derived from the synchronizing impulses and has a value which is a measure of their height. In .my improved system, however, the gain control voltage is derived from a point in the receiver where the D.-C. component is in the picture signal. Preferably, this voltage is derived from the second detector output in a manner which will be described hereinafter.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figures 1 and 2 are curves which are referred to in describing the invention, and

Figures 3, 4 and 5 are circuit diagrams of several embodiments of my invention.

The characteristics of the transmitted signal O paratus, and assigned to the Radio Corporation upon which my invention is dependent are illustrated in Figs. 1 and 2 which show a carrier wave modulated by a light picture signal (light background) and by a dark picture signal (dark background), respectively. The modulating video signal, of course, is the envelope of the carrier. The horizontal synchronizing impulses which occur at the end of each scanning lineare indicated at I.

In the specific case shown, each synchronizing impulse consists of a blanking impulse lb which is referred to as the pedestal and a super-sync impulse la which is set on top of the pedestal. The top of the pedestal always goes to a certain level, in the signal transmitted, such as the black level or a certain amount beyond black. A system for producing picture and synchronizing signals of this character is described and claimed in Bedford Patent 2,192,121, issued February 27, 1940, entitled Television systems and the method of operation thereof, and assigned to the Radio Corporation of America.

The picture signals and the synchronizing impulses may be transmitted with the D.-C. component included by employing D.-C. amplifiers at the transmitter or, preferably, by employing a system of the character described and claimed in Bedford application Serial No. 203,015, filed April 20, 1938, entitled Picture transmitting apof America.

As indicated in Figs. 1 and 2, the so-called negative modulation by synchronizing impulses is employed. In other words, the synchronizing impulses (which are in the dark direction) produce maximum amplitude of the carrier wave.

A comparison of Figs. 1 and 2 shows that a change from a dark picture toa light picture does not cause a change in the amplitude or level of the synchronizing impulses. It follows that any variation in the height of the synchronizing impulses (or a variation in the amplitude of the carrier representative of synchronizing impulses) will be a result of fading or attenuation of the transmitted signal.

A gain control voltage which has a value depending upon the height of the synchronizing impulses, and which is independent of the background of the picture being transmitted, may be derived as shown in Fig. 3.

Referring to Fig. 3, my invention is shown applied to a television receiver of the superheterodyne type. The receiver comprises a first detector 6, a tunable oscillator l and an intermediate frequency amplifier 8 which usually consists of a plurality of amplifier stages.

The modulated I. F. carrier, which is of the character shown in Figs. 1 and 2, is supplied from the I. F. amplifier 8 through a tuned I. F. transformer 9 to the second detector H.

Preferably, the second detector II is of the push-pull type comprising diodes 2 and H). In this particular circuit the cathodes of the diodes l2 and it are connected to opposite terminals of the secondary M of transformer 9 while the plates of the diodes l2 and I3 are connected through a resistor iii to the mid-point of secondary I l.

The video signal appears across the resistor I6 and is supplied to a video amplifier, the first stage of which is indicated at H, and then impressed upon a cathode ray tube (not shown).

Video signal is also supplied from the amplifier H to an amplifier and separating circuit I3 which removes the picture signal and supplies only the synchronizing impulses to the deflecting circuits (not shown) of the cathode ray tube.

In order to obtain A. V. C., the video signal from the econd detector H is impressed upon an A. V. C. circuit comprising a diode 2| having the usual indirectly heated cathode 22 and a plate 23. The cathode 22 is connected to the plate end of the resistor I6 while the plate 23 is connected to the cathode end of resistor l6 through a resistor 24 and ground. Resistor 25 is shunted by a condenser 26.

The impedance values of resistor 24 and con denser 26 are such that the diode 2| functions as a peak voltage indicator whereby there appears across the resistor 24 a D.C. voltage or slowly varying voltage which is a measure of the peak amplitude of the synchronizing impulses and, therefore, of the attenuation of the received signals. Specifically, the time constant of the resistor 24 and condenser 26 in series is such that the condenser 25 discharges only a slight amount between the occurrence of successive horizontal (high frequency) synchronizing impulses.

The A. V. C. voltage is upplied to the several stages of the I. F. amplifier 8 through a conductor 21 which is connected to the negative end of resistor 24. It will be understood that the A. V. C. voltage may be impressed upon the control grids or other gain control electrodes of any desired amplifier stages as is well understood in the art.

The push-pull second detector may, of course, be replaced by a single-sided detector if desired. A similar substitution may be made in the circuits which are described hereinafter.

In the circuit of Fig. 3 and in the circuits described hereinafter, the receiver cannot block when the modulation goes off the carrier and comes back on because the D.C. produced by the unmodulated carrier (which D.C. is a means of the carrier amplitude) flows through the A. V. C. diode 2| and resistor 24, thus holding down the gain.

If D.C. from the unmodulated carrier did not go through the A. V. C. resistor 24, the receiver would be wide open (at maximum gain) when the modulation went off. Then, when modulation came on and if the signal were strong, the I. F. amplifier tubes would be overloaded, the sync would be distorted or wiped off, and the A. V. C. would remain ineffective whereby the gain would not be reduced and the tubes would continue to be overloaded.

In Fig. 4 there is shown another embodiment of my invention which gives better gain control than the circuit of Fig. 3 and which has a delayed action. Like parts in Figs. 3 and 4 are indicated by the same reference numerals.

In the circuit of Fig. 4 an amplifier 29 has been included in the A. V. C. circuit in order to increase the gain control. Since this amplifier reverses the polarity of the gain control voltage, the diode comprising resistor 25 and condenser 25 is placed in the diodes cathode circuit rather than in its plate circuit, and the plates and cathodes of the second detector are so connected that the synchronizing impulses are positive at the junction point of the resistor it and the cathodes of diode l2 and I3.

The desired delay before the A. V. C. circuit begins to control may be obtained by connecting the cathode 22 through resistor 24 and a switch 25 to a more negative point (53 volts) on a voltage divider 3| or the like than the point (-56 volts) to which the plate end of resistor is is connected. Thus, in the specific example given, the plate 23 of diode 2| must have more than +3 volts applied thereto from the second detector before the diode 2| draws current. Preferably, however, the switch 25 is in the position illustrated whereby the 3 volts delay is applied to the grid of the tube 29. Thus, assuming tube 29 is biased beyond cut-off at 3 volts on the grid, a voltage of 3 volts across resistor 24 is required before plate current flows in the tube 29.

When the voltage of the synchronizing impulses appearing across resistor IE is above the delay level (3 volts in the example given), a D.C. gain control voltage appears across resistor 24 and is amplified by the D.C. amplifier 2E. This amplified voltage is supplied from the plate circuit of amplifier 29 to the I. F. amplifier 6 through a conductor 32. Preferably, a condenser 33 is connected across the plate resistor of amplifier 28 to filter out any I. F. carrier that may be present.

It will be noted that the plate or low potential end of resistor I6 is maintained at ground potential for I. F. frequencies by means of a condenser 34.

It will be understood that the impedance values for the elements 24 and 26 in Fig. 4 are determined as explained in connection with Fig. 3.

As illustrated, the synchronizing separator l8 may be supplied from the output of the detector I! rather than from the output of the amplifier II.

An additional advantage of connecting the A. V. C. diode 2| to a point in the video channel where the direct current has not been lost, preferably to the second detector output, is that the D.C. voltage applied to this diode corresponds to the height of the synchronizing impulses as measured from the zero axis of the carrier wave (Figs. 1 and 2). That is, voltage starts to build up across condenser 25 as soon as voltage appears across resistor IB. Thus a higher Voltage is applied to the diode than would be the case if the synchronizing voltage had to reach a predetermined value before current flowed through the diode 2| and, as a result, the diode output is greater because it is being operated on a higher and comparatively straight portion of its characteristic.

In Fig. 5 there is shown an embodiment of my invention which provides delayed A. V. C. and which utilizes a single vacuum tube which is a combination diode-triode. Like parts in Fig. 5 and in the preceding figures are indicated by the same reference numerals.

Referring to Fig. 5, the A. V. C. tube includes a triode comprising a cathode 4|, a control grid 42 and an anode 43. It also includes diode plates M and 25 associated with the cathode 4!.

Voltage is supplied to the triode plate 43 from a positive point on a voltage divider 45.

The cathode M is connected to a negative point on the voltage divider 45 through a resistor while the diode plate 46 is connected to a less negative point on the voltage divider 45 through a resistor 49.

The video signal from the detector ll is impressed across the diode 44-4I through a conductor 41 and the resistor 24, the polarity of the signal being such that the synchronizing impulses cause current flow through the diode to charge the condenser 26 in shunt to resistor 24. Thus an A. V. C. voltage appears across resistor 24.

The voltage across resistor 24 is applied to grid 42 through conductor 48 whereby the plate current of the triode section is reduced. This causes a reduction in the voltage drop across the cathode resistor Sl whereby the cathode M goes less positive.

The plate voltage on the triode plate 33 is adjusted to such a value that when there is no in coming signal the cathode Q! is more positive than the diode plate 46 by an amount corresponding to the desired delay in the A. V. C. action.

As above stated, an incoming signal makes the cathode 4! less positive. If the signal exceeds a predetermined amplitude, the cathode M goes negative with respect to the plate 46 and current flows through resistor 39 whereby an amplified A. V. C. voltage appears thereaoross and is supplied to the I. F. amplifier through a conductor 52. Thus, the gain of the I. F. stages is reduced in accordance with the increase in amplitude of an incoming signal.

The resistor 49 preferably is shunted by a condenser 53 for filtering out any I. F. carrier that may be present. Also, the plate end of resistor Id of the second detector is held at ground potential for I. F. signals by means of the condenser The cathode 4| of the diode-triode is likewise held at ground potential for I. F. signals by means of a condenser 5 The impedance values of resistor 24 and condenser 26 are determined as explained in connection with the preceding figures.

In the circuit of Fig. 5, as in the circuits previously described, an increase in the carrier strength causes an increase in the amplitude of the synchronizing impulses whereby the amplitude of the negative A. V. C. voltage is increased and the gain of the I. F. amplifier is reduced.

In the several figures, the usual by-pass condensers for the voltage divider have been omitted in order to simplify the drawings.

I claim as my invention:

1. In a television receiver for the reception of a carrier wave modulated by a composite sign=al comprising picture signals and their direct current component and periodically recurring synchronizing impulses wherein said impulses are of greater amplitude than picture signals of like polarity and wherein said impulses modulate said carrier wave in the direction to make the carrier wave of maximum amplitude representative of said impulses, means for intercepting said modulated carrier wave, means for dem-odulatin-g said carrier wave to produce a demodulated signal including said composite signal and said direct current component, an amplifier connected in cascade with said demodulator, a diode and a resistor in series with each other to form a series combination, a condenser in shunt to said resistor, means for impressing said demodulated signal including said direct currentcomponent across said series combination with the correct polarity to make said impulses produce a periodic flow of current through said diode to charge said condenser, said resistor and said condenser in series having a time constant such that said condenser discharges only a slight amount between successive synchronizing impulses whereby said diode passes only the peaks of said impulses, and means for controlling the gain of said amplifier in accordance with the resulting A. V. C. voltage appearing across said resi'stor.

2; The invention according to claim 1 characterized in that said last means includes a direct current amplifier comprising a vacuum tube which is biased beyond cut-cit for the condition of no signal input by an amount corresponding to the amount of delay desired in the A. V, C.

action.

3. The invention according to claim 1 characterized in that said diode and said resistor in series are connected directly across the output circuit of said demodulating means.

4. In a television receiver for the reception of a carrier wave negatively modulated by a composite signal comprising picture signals and their direct current component and periodically recurring synchronizing impulses wherein the synchronizing impulses are of greater amplitude than picture signals of like polarity, means for intercepting said carrier wave, an amplifier, a demodulating means connected in cascade with said amplifier, said demodulating means being of the diode type including an output resistor whereby said picture signals and said synchronizing impulses appear across said resistor, a diode and a second resistor connected in series with each other and direct current connected across said first resistor in the correct direction to rectify said impulses, a condenser connected across said second resistor, said second resistor and said condenser having such values that the condenser holds a large percentage of its charge between the occurrence of successive synchronizing impulses whereby only the peaks of said impulses are rectified and whereby a gain control voltage appears across said second resistor, and means for controlling the gain of said amplifier in accordance with said gain control voltage.

5. In a television receiver for the reception of a carrier wave negatively modulated by a composite signal comprising picture signals and their direct current component and periodically recurring synchronizing impulses wherein the synchronizing impulses are of greater amplitude than picture signals of like polarity, means for intercepting said carrier wave, an amplifier having a gain control electrode, a detector connected in cascade therewith and comprising a diode and a resistor connected in series with respect to the source of modulated carrier wave whereby said composite signal appears across said resistor, a second diode and a second resistor connected in series with each other and direct current connected across said first resistor, said second diode having a plate and a cathode, means for maintaining said plate negative with respect to said cathode at a predetermined voltage during the condition of no signal input, a condenser connected across said second resistor and having sufficient capacity to hold a high percentage of its charge between the occurrence of successive synchronizing impulses whereby the voltage across said second resistor is a measure of the average of the peak amplitudes of said impulses, and a direct current amplifier having its input electrodes connected across said sec ond resistor and having its output circuit connected to said gain control electrode through a direct current connection.

6. In a television receiver for the reception of a carrier wave modulated by a composite signal comprising picture signals and periodically recurring synchronizing impulses wherein the synchronizing impulses are of greater amplitude than picture signals of like polarity, means for intercepting said carrier wave, an amplifier and a detector connected in cascade, said detector having an output resistor across which appears said composite signal, a vacuum tube comprising a cathode, a grid and a plate, and further comprising a first diode plate and a second diode plate associated with said cathode, a direct current connection between one end of said resistor and said first diode plate, a direct current connection between the other end of said resistor and. said cathode, said last connection including a gain control resistor shunted by a condenser having sufficient capacity to hold a large percentage of its charge between the occurrence of successive synchronizing impulses, a resistor connected between said cathode and a point of negative potential on a voltage source, said first plate being connected to a positive point on said source, a connection for impressing the potential appearing across said gain control resistor upon said grid whereby said cathode goes more negative in response to an increase in the amplitude of an incoming signal, a resistor connected between said second diode plate and a point on said voltage source which is more positive than said point of negative potential by an amount equal to the desired delay in the" automatic volume control action, and a connection between a point on said last resistor and said amplifier for controlling the gain of said amplifier in accordance with the voltage appearing across said last resistor.

CIRO C. MARTINELLI. 

